This invention relates generally to apparatus for treating liquids wherein a liquid-solid is separated in an arcuate sedimentation zone and more specifically to an improved means for collecting and removing settled solids from such a zone.
In the treatment of small flow of liquids such as sewage and other wastewater streams, the prior art has commonly employed integral circular plants, i.e. plants in which all components are enclosed in a single outer wall. Such configuration possess numerous fabricational advantages; for example, the cost of material and fabrication are lower for a relatively small integral treatment plant than for a plant comprising physically separate elements. Moreover, integral plants are compact and require a small land area for installation; such a plant also has a potential for much more simplified overall design as compared to a non-integrated facility. By providing a minimum perimeter to cross sectional area ratio, circular design tends to minimize material requirements for fabrication of the integral plant while promoting a highly efficient component arrangement. Additionally, construction costs may be less in some instances for circular geometries than for other shapes, as for example in concrete fabrication.
The prior art has employed biological treatment processes in small circular plants, primarily because of their applicability to a wide variety of wastewaters and effluent requirements and comparatively low capital cost. A particularly efficient integral circular plant for activated sludge treatment of wastewater by aeration with at least 50% oxygen gas is disclosed in U.S. Pat. No. 3,890,231, issued June 17, 1975 to L. M. LaClair et al. In this apparatus, an inner circular wall is provided, concentric with the outer circular wall, to provide an inner treatment volume and in intermediate volume between the inner and outer walls. At least two separate oxygen aeration zones are provided in the plant, together with an arcuate clarification zone in the intermediate volume, bounded by radial partitions and segments of the concentric inner and outer walls serving to define arc lengths of between 90.degree. and 330.degree.. In operation, the oxygenated liquor from the final aeration zone is directed to means for uniformly distributing the same in the arcuate sedimentation zone around the inner wall segment thereof, and the oxygenated liquor flows radially outward from the inner wall to the outer wall. In this manner, the sedimentation path, i.e., the liquor flow path length required for sedimentation, is contained within the actual provided radial flow path, because of the beneficial radial expansion of liquid in the clarifier.
The provision of an arcuate clarification zone in the above-described apparatus has been shown to provide unique advantages for the integral wastewater treatment plant relative to the designs of the prior art. Because the arcuate clarification zone occupies only part of the intermediate volume of the plant, the remaining peripheral portion of the plant may be employed for other purposes, as for example, aeration, digestion of activated sludge, and chlorination of the clarifier effluent. Such provision thus imparts a flexibility to the arcuate clarification zone integral circular plant which is not achieved in plants having a fully extended peripheral clarifier or a central circular clarification zone.
Despite the foregoing advantages of the arcuate clarifier integral circular plant, certain disadvantages have become apparent as the system has been in use, relating to the means heretofore employed for collection and removal of settled solids from the bottom part of the arcuate clarification zone. These means generally comprise a series of sludge pick-up heads transversely spaced across the arcuate clarifier and positioned near the floor thereof. The pick-up heads are respectively supported by hollow shafts for flow therethrough to an overhead sludge trough and an air lift blower is joined through connecting lines to the lower ends of each of the respective shafts. This pick-up assembly is mounted on a motorized radially extending bridge which is continuously and repetitively moved around the clarifier along an arcuate path to provide continuous removal of settled sludge from the floor of the clarifier.
While the foregoing sludge collection and removal device is generally satisfactory in operation involving the treatment of liquids wherein the liquid-solid to be separated in the arcuate clarification zone comprises a relatively fine dispersion of suspended solids, the device is particularly susceptible to clogging and plugging of the solids conveying passages when large sized solids and extraneous solid matter are present in the liquid-solid introduced to the clarification zone. For example, in the treatment of municipal sewage, it is frequently desirable to operate the wastewater treatment system without a primary sedimentation step in order to minimize capital equipment and operating costs which would otherwise be associated with primary sedimentation apparatus. In this case, the heavy material such as garbage and stringy material such as rags which is present in the influent to the integral circular plant passes through the pre-treatment and aeration steps and is introduced to the sedimentation zone where it occludes the sludge pickup heads and transfer conduits. Such occurrence renders the integral circular plant inoperative, requiring costly shutdown and cleaning of the solids collection and removal device before treatment can be resumed.
Another problem associated with the aforedescribed solids collection and removal device in the arcuate clarifier is that the radial bridge and associated pickup head assembly, being structurally characterized by some significant dimensional width with an essentially rectangular geometry, is not able to fully traverse the non-rectangular areas adjacent the radial partition and walls of the arcuate zone. This results in the formation of "dead space" in such areas and the occurrence of solids accumulation which significantly reduces the overall efficiency of the integral plant. The provision of a design of the solids collection and removal device to overcome such problems would involve major and extensive structural alteration of the basic apparatus configuration which would significantly increase the cost and complexity of the assembly. Furthermore, it is not generally desirable to provide non-radial partition end walls in the arcuate clarification zone such as would yield near-rectangular areas at the end walls which would accommodate the sludge collection and removal device. This is because the end walls also form the bounding walls of the treatment zones adjacent to the clarification zone in the intermediate volume such as aeration or mixing zones. These zones require a geometry free from sharp corners or regions inaccessible to the liquid flow which would cause stagnant areas and short-circuiting and thus require radial partition bounding walls.
A still further problem encountered with the aforedescribed sludge collection and removal assembly is that when the feed liquid-solid is introduced in the clarification zone at the inner wall segment and flowed radially outwardly to the outer wall segment, such as is highly beneficial in achieving good sedimentation efficiency, a settled sludge gradient exists between the inner and outer wall segments of the clarification zone. Such variation in solids deposition requires the addition of control means to the solids collection and removal device to provide corresponding variation in volumetric flow rates of sludge withdrawal for the multiple pickup heads positioned between the inner and outer wall segments and thereby assure a suitably high solids concentration in the withdrawn sludge. Nonetheless, the additional control means again increase both the cost and complexity of the overall device.
In contrast to the problems encountered with sludge collection and removal in the arcuate clarification zone, the prior art has been able to employ comparatively simple and less costly devices for sludge collection and removal in non-arcuate rectangular and circular clarifiers and sedimentation basins. For example, it has been found particularly advantageous in these non-arcuate configurations to employ scraping devices featuring scraper blades which traverse the bottom of the sedimentation basin and move the settled solids to a localized sump or trough means from which the collected solids are withdrawn.
Prior art solids collection and removal devices utilizing scraper blades can be typified by two basic designs -- those in which a scraper blade is supported by a reciprocating bridge which moves along a longitudinal, e.g., rectangular, tank and those in which the bridge moves continually along an orbital path about the axis of a circular tank or around a central circular dividing wall around a fully extended peripheral sedimentation zone.
Those devices designed for reciprocation motion in a longitudinal basin cannot generally be applied to an arcuate sedimentation zone without the formation of the aforedescribed dead space adjacent the end walls of the zone. Such devices are characteristically constructed with the scraper blade rigidly joined to an overlying bridge assembly as by means of vertically extending beam or spar members and arranged to direct settled solids to a central location such as a solids trough comprising a central alley of the longitudinal basin. In these designs the scraper blade is typically disposed at an angle with respect to the transversely extending bridge so as to effectively direct settled solids into the solids collection trough. In application to an arcuate sedimentation zone, such designs result in the existence of large unscraped areas in the end wall regions of the zone, with the aforedescribed deleterious consequences. It has been proposed by the prior art to overcome such difficulty in the arcuate sedimentation zone by positioning of transversely extending solids collection troughs adjacent the end walls of the zone. However, this expedient is not suitable in practice inasmuch as the circumferential length of the sedimentation zone over which the scraper must operate is very long and in consequence fine sludges tend to be undesirably stirred up and to re-enter suspension during the long traverse of the scraper blade between the end wall regions, thereby lowering solids collection and removal efficiency for the system. In addition, operation in this manner may place heavy mechanical load on the bridge/scraper assembly which is most desirably avoided in practice to achieve a simple and inexpensive structural configuration.
On the other hand, the devices utilizing scraper blades which have heretofore been employed in circular tanks and in fully extended, i.e. 360.degree., peripheral sedimentation zones are characteristically designed for uni-directional rotation. In these devices the scraper blades are rigidly connected to the overlying bridge or supporting boom and cannot accommodate solids removal in the reverse direction. Thus, such devices are fundamentally inapplicable to an arcuate configuration sedimentation zone bounded by end wall partitions, in which the solids collection and removal function is most desirably carried out by a device which moves continuously and reciprocatingly across the arcuate zone between the end wall partitions and is active in removing solids from the zone in both directions of movement.
Accordingly, it is an object of the present invention to provide an improved means for collection and removal of settled solids from the bottom part of an arcuate sedimentation zone which overcomes the aforementioned difficulties.
It is another object of the present invention to provide an improved means of the above type featuring a scraper blade which is mechanically simple and inexpensive to fabricate.
Other objects and advantages of the present invention will be apparent from the ensuing disclosure and appended claims.